1. Field of the Invention
The present invention relates to a magnetic recording medium which is improved in electromagnetic conversion characteristics, running property and running endurance.
2. Description of Prior Art
Magnetic recording media have been widely employed in recent years as an audio tape, a video tape or a magnetic tape for a computer system. The magnetic recording medium is desired to be excellent in various properties such as electromagnetic conversion characteristics, running property and running endurance.
For enhancing the electromagnetic conversion characteristics of the medium, for example, a method of improving a coercive force (Hc) of a ferromagnetic powder to be employed has been utilized, and recently a ferromagnetic powder having a coercive force of not less than 500 Oe is generally employed.
Further, a magnetic recording medium such as an audio tape or a video tape has been broadly used in the form of a cassette encasing the tape therein. In the case of such tape encased in a cassette (i.e., cassette tape), a demand for longer time recording has increased, and hence the tape is apt be made thinner. A magnetic recording medium generally comprises a nonmagnetic support and a magnetic recording layer provided on the support, so that it is thought that the thickness of the magnetic recording layer is made smaller for the purpose of making the magnetic recording medium as a whole thinner. However, if the thickness of the magnetic recording layer is made smaller, an adverse effect is given to the electromagnetic conversion characteristics of the resulting medium. Accordingly, the thickness of the nonmagnetic support is generally made smaller for that purpose.
The nonmagnetic support functions not only to give a mechanical or physical strength to the magnetic recording medium in the longitudinal and width directions but also to absorb a shock caused by a pressure applied to the recording medium in the depth direction. That is, the nonmagnetic support absorbs a shock given to the magnetic recording layer when the recording layer is brought into contact with a magnetic head, whereby the recording layer is prevented from suffering damages caused by the contact with a magnetic head. For this reason, as the thickness of the nonmagnetic support is made smaller, the function of shock absorption of the nonmagnetic support decreases. Especially in recent days, it is required for a thin nonmagnetic support to provide a high strength to the magnetic recording medium, so that a resin film having high mechanical or physical strength is widely employed. However, such resin film is generally rigid, and hence the nonmagnetic support of such resin film is liable to be decreased in the above-mentioned function of shock absorption.
By the decrease of the function of shock absorption of the nonmagnetic support, there arises a problem that a ferromagnetic powder easily separates (drops off) from the magnetic recording layer to deposit on a magnetic head when the recording layer is brought into contact with the head, whereby running endurance of the recording medium is deteriorated.
A magnetic recording medium having a recording layer in the form of two layers has been already known, and the known magnetic recording layer of two-layer type generally comprises a flexible (soft) upper layer and a lower layer of high hardness. That is, in the magnetic recording medium having such structure, the upper layer of the magnetic recording layer which is to be in contact with a magnetic head is made flexible, and thereby the magnetic recording layer can be enhanced in easiness of close contact with the head.
However, it has been confirmed by the present inventors that the above-mentioned magnetic recording medium having a two-layer type magnetic recording layer including the upper flexible layer and the lower layer of high hardness decreases in running property and running endurance, and additionally the recording medium shows poor bonding strength between the nonmagnetic support and the magnetic recording layer. Also confirmed is that a magnetic tape having such constitution of the magnetic recording layer shows an extreme variation of electromagnetic conversion characteristics (i.e., variation of output level) when stored under condition of high temperatures such as a temperature of higher than 60.degree. C. It is presumed that the variation of electromagnetic conversion characteristics in the storage at high temperatures takes place for the following reason. When the magnetic tape is stored in the wound state at a high temperature, the upper layer of the magnetic recording layer is made more soft by the high temperature, and the protruded and depressed portions of the surface of the nonmagnetic support of the magnetic tape are copied onto the surface of the upper layer of the magnetic recording layer. As a result, electromagnetic conversion characteristics of the medium varies in the reproduction procedure.
Japanese Patent Provisional Publication No. 57(1982)-78630 discloses a magnetic recording medium comprising two magnetic recording layers in which the orientations of the ferromagnetic powders contained in the two recording layers are made different from each other, whereby the ratio of the elasticity modulus between in the longitudinal direction and in the width direction of the medium is in the specific range. The magnetic recording medium is improved in strength particularly in the edge portion. However, the recording medium is not improved as far as the electromagnetic conversion characteristics, running property and running endurance are concerned.